In recent years, along with a rapid development of communication networks, frequency range to be used for the communication is extended to cover high-frequency region such as microwave region or millimeter wave region. With regards to the dielectric ceramic composition for high frequency, it is demanded that a dielectric resonator using dielectric ceramics obtained by sintering the dielectric ceramic composition has a large unloaded Q-value. Further, the dielectric ceramic composition for high frequency is demanded to have a small absolute value in temperature coefficient τf of resonance frequency f0. On the other hand, as the dielectric constant εr of the dielectric ceramics is larger, a microwave circuit or millimeter wave circuit can be more reduced in the size. However, in terms of high-frequency region corresponding to microwave region, when the dielectric constant εr becomes too large, the circuit is excessively reduced in the size, with the result that high processing precision is demanded. Therefore, a material having a small dielectric constant εr is required.
As the dielectric ceramic composition for manufacturing a dielectric resonator having a large Q-value and a small absolute value in temperature coefficient τf of the resonance frequency f0, BaO—MgO—WO3-base material (refer to JP(A)-6-236708 (paragraph number [0033] on page 11, tables 1 to 8)), MgTiO3—CaTiO3-base material (refer to JP(A)-6-199568 (paragraph number [0018] on page 5, tables 1 to 3)), and the like have been proposed. However, the dielectric constant εr of the dielectric ceramics obtained from the above ceramic compositions exceeds 10. Hence, the dielectric ceramic composition from which dielectric ceramics having a lower dielectric constant can be manufactured is demanded.
Forsterite (Mg2SiO4) and Alumina (Al2O3), which have relatively small dielectric constants εr of 7 and 10 respectively, are known as the dielectric ceramic composition from which a dielectric ceramics excellent in high-frequency characteristics can be manufactured. However, the temperature dependency (τf) of resonance frequency is large on the minus side (−60 ppm/° C.), so that an application to such uses as the dielectric resonator and dielectric filter where the temperature dependency needs to be small is limited.
In recent years, laminated ceramic parts formed by laminating dielectric ceramics, such as a laminated dielectric resonator, a laminated dielectric filter, or a laminated dielectric substrate have been developed and the lamination by the simultaneous sintering of a dielectric ceramic composition and an internal electrode is being performed. However, the above-described dielectric ceramic compositions have a difficulty in performing the simultaneous sintering with the internal electrode because of their high sintering temperature of 1300° C. or more and therefore, for forming a lamination structure, material of the internal electrode is limited to an expensive high-temperature resistant material such as platinum (Pt). For this reason, there has been demanded a dielectric ceramic composition capable of performing the simultaneous sintering with the internal electrode at a low temperature of 1000° C. or less, using as the internal electrode material silver (Ag), Ag—Pd, Cu and the like, which are low resistant and inexpensive conductors.
As the dielectric ceramics having a small dielectric constant and capable of performing the sintering at a low temperature, ceramics comprising a ZnAl2O4 crystal, an α-SiO2 crystal, a Zn2SiO4 crystal, and a glass phase is known (refer to JP(A)-2002-338341 (paragraph number [0050] on page 10, table 4, etc.)). This material is a printed circuit board material including the glass phase and, therefore, a mechanical strength is stressed in it. However, the Q-value of the resonator is not sufficient for a high-frequency dielectric ceramics. Further, there is no description about the temperature coefficient τf of resonance frequency f0 in the above publication.
In addition, as the dielectric ceramics having a small dielectric constant and capable of performing the sintering at a low temperature, a ceramics comprising SiO2, Al2O3, MgO, ZnO, and B2O3, where crystalline phases of ZnO and Al2O3, crystalline phases of ZnO and SiO2, crystalline phases of MgO and SiO2, and amorphous phase of SiO2 or amorphous phases of SiO2 and B2O3 are present together is known (refer to JP(A)-2002-53368 (paragraph number [0053] on page 5, table 2, etc.)). This material is a printed circuit board material including the glass phase and, therefore, a mechanical strength is stressed in it. However, the Q-value of the resonator is not sufficient for high-frequency dielectric ceramics. Further, there is no description about the temperature coefficient τf of resonance frequency f0 in the above publication.